It just sort of happened, don't know why. The editor just won't update GM.

[Edit] Turns out GMS isn't removing anything, it just doesn't show the code I've written in GMEdit and when I edit something in that script and save it it removes everything written in GMEdit.

GMS1 doesn't watch for file changes at all - if you opened some script/object in GMS1, edited it, and saved, it'll likely just show that version next time you open it instead of checking what's on the disk. Not sure if there's any way to correct this aside of reloading project - maybe it'd care when source control is enabled?

I can confirm that GMS1 does reload changed files when you use source control.

Thank you all!